Prenatal Tests

During your first prenatal visit, your provider will recommend a number of tests to screen you for complications that could develop during pregnancy and delivery.

Blood Type

Determining your blood type is important because when your baby's blood type is different from yours, the baby has a slightly increased risk of developing jaundice after delivery. The baby's blood type is usually determined from a blood sample taken from the umbilical cord at birth. If your baby's blood type doesn't match yours, your provider will closely watch your baby for signs of jaundice.

Did you know—

There are four different blood types: O, A, B, and AB. The blood types differ in the antigens (proteins) found on the surface of the red blood cells.

Rh Antibody Screen

Rh is another antigen on red blood cells that has significance during pregnancy and delivery. Women who don't have the Rh protein (are Rh-negative) can become sensitized to the Rh antigen and develop an immune response (make antibodies) against the Rh antigen when they are exposed to Rh-positive red blood cells. If the fetus is Rh-positive, the presence of these maternal antibodies can cause serious problems. The maternal antibodies can cross the placenta, and attack and destroy the fetus's red blood cells, causing severe anemia. Severe anemia can cause erythroblastosis fetalis (fetal hydrops), a syndrome characterized by heart failure, diffuse edema (swelling), ascites (fluid in the abdomen), and pericardial effusion (fluid in the sac around the heart).

If a woman is Rh-negative, but has not developed Rh antibodies, her doctor will try to keep her from becoming sensitized and making antibodies. A woman who is Rh-negative should receive Rh immune globulin (RhoGAM, an intramuscular injection containing the Rh antibodies) if there is a chance that her baby could be Rh-positive because the father is Rh-positive (or when the father is unknown). So the immune response doesn't cause problems in future pregnancies, women in this situation should receive RhoGAM if they experience:

early pregnancy loss;

vaginal bleeding during pregnancy; or

invasive procedures (for example, amniocentesis) in which maternal and fetal blood could mix.

In addition, women who are Rh-negative should receive RhoGAM at 28 weeks gestation if there is a chance the baby is Rh-positive and after delivery if the baby is found to be Rh-positive.

The Sensitized Rh-Negative Woman

If the antibody screen for Rh comes back positive during the first prenatal visit, the antibody titer (see below) is checked as well. Titers greater than 1:8 are concerning to your doctor, and he or she will treat you and your fetus as if erythroblastosis fetalis (fetal hydrops) is a risk. From that point on, your doctor will take frequent surveys of the fetus to look for pericardial effusion (fluid in the sac around the heart) and ascites (fluid in the abdomen). To determine the severity of disease, your doctor will do an amniocentesis with fluid analysis, along with an analysis of the fetus's blood in order to measure anemia.

Antibody Screen

The antibody screen test determines whether a patient has developed antibodies to blood cell antigens. Many antibodies detected on this screen do not cause adverse pregnancy outcomes, but some antibodies can indicate risks for complications. The Rh antibody, for example, is detectable by this test. If present, it is reported as a titer such as 1:4.

Hematocrit and Complete Blood Cell Count (CBC)

Hematocrit is the percentage or concentration of red blood cells in the blood and is used as an indicator for anemia. The most common cause of anemia in younger women is iron deficiency; if a pregnant woman has anemia, often her doctor will prescribe iron tablets (for example, Ferro-Sequels, ferrous sulfate) in addition to prenatal vitamins (for example, Prenate Advance). Less common causes of anemia include beta-thalassemia, sickle cell disease, and folate (folic acid) deficiency.

During pregnancy the hematocrit value typically decreases as the overall blood volume increases and the non-liquid components of blood become more diluted. A hematocrit of less than 35 is generally considered to be an indicator of anemia.

Some physicians obtain a CBC (complete blood count). A CBC measures many components of the blood including white blood cells, platelets, hematocrit, and hemoglobin (the part of the blood that carries oxygen).

Rubella

Rubella infection during pregnancy can cause serious problems in the fetus. However, if your rubella antibody status is unknown, your blood should be tested for the presence of protective antibodies. Most women in the have been vaccinated against rubella and are thus immune to the infection.

If the test shows that you are not immune to rubella, you should receive the vaccine after delivery and avoid exposure to children with fever and rash during your pregnancy.

The vaccine is not given during pregnancy because it can sometimes cause low-grade infection. If you inadvertently received a rubella vaccine before you knew you were pregnant, tell your provider. She or he may refer you to a geneticist or a maternal-fetal medicine specialist for counseling. There are no known reports of congenital rubella syndrome in babies born to women who received the vaccine during pregnancy.

Rapid Plasma Reagin Test (RPR Test)

The RPR test is a screening test for syphilis. Individuals who have been infected with T. pallidum (the bacteria that causes syphilis) will have a positive reaction to this blood test, regardless of whether they have been adequately treated. In addition, some individuals who have never been infected with the syphilis bacteria will have a positive reaction—this is called a false-positive result. In order to determine whether the result is a true positive or a false positive, most laboratories will perform a confirmatory test called the FTA-ABS (Fluorescent Treponemal Antibody-Absorbed) test. The FTA-ABS test more accurately detects the presence of syphilis infection.

Because syphilis can affect the fetus, treatment of a woman who tests positive during pregnancy is very important. In many states, an RPR test is repeated during the third trimester or on admission to the hospital.

If you have a false-positive RPR (that is, the RPR is positive and the FTA-ABS is negative), your provider may evaluate you for other rheumatoid conditions that can cause a false-positive result (for example, systemic lupus erythematosus). The majority of women with a false-positive RPR, however, have no underlying condition.

Hepatitis B Surface Antigen

Your provider will most likely screen you for hepatitis B at your first prenatal visit. If you have never been vaccinated for hepatitis B, then you will be tested for the presence of Hepatitis B antibodies. If you have been vaccinated, then you will be tested for the hepatitis B surface antigen. If the antigen test is positive, regardless of any other results, you are potentially infectious. In this case, soon after birth your baby will receive the normal vaccine for hepatitis B as well as hepatitis B immunoglobulin (HBIG) to prevent active hepatitis.

Human Immunodeficiency Virus (HIV)

Many physicians offer the HIV antibody test to all pregnant patients. If you have any possible risk for HIV, it's a good idea to take this test because receiving anti-viral therapy during pregnancy and labor reduces the rate of transmission to the infant.

Gonorrhea and Chlamydia

Gonorrhea and chlamydia are sexually transmitted diseases (STDs) that can cause infection in the fetus and preterm labor. Chlamydia is a very common genital infection. Screening tests may be performed with the use of a vaginal swab or a urine specimen. These infections are easily treated with oral antibiotics.

Papanicolaou (PAP) Smear

This screening test for cervical cancer involves scraping a small amount of cells from the surface of the cervix during a pelvic exam. Because the cervix is softer and more vascular during pregnancy, a PAP smear may cause some mild vaginal bleeding (spotting).

GBS (Group B Streptococcus)

Many women carry the group B streptococcus in the vagina or rectum. Some physicians screen for GBS by taking a culture of the vagina and rectum at about 34 weeks of pregnancy. GBS may also be detected on urine culture. Other physicians do not screen for GBS, but will treat women who have risk factors, such as prolonged rupture of membranes (PROM) or preterm labor. Women who test positive or who have risk factors are treated with antibiotics during labor to prevent the rare occurrence of pneumonia and overwhelming infection in the newborn.

Urinalysis

At each prenatal visit, your provider will collect a sample of urine and will perform a urine dipstick test, which can test for the presence of protein—protein in the urine could indicate the presence of preeclampsia. If protein is present, you will have further studies to detect the cause of the abnormality.

Other dipstick tests that may be performed check for leukocyte esterase or nitrates, substances that may be present when there is a urinary tract infection (UTI). Pregnant women are more susceptible to urinary tract infections and may be asymptomatic (without symptoms). Your provider will aggressively treat any bladder infection to prevent the development of a kidney infection.

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